The present invention is related to the production of eyeglasses temples. In more detail it relates to the production of eyeglass frames and part of them made of specially processed NiTi shape memory alloys. These frames have a high degree of elasticity and they can be used in a broad range of temperatures, in particular in the range of from xe2x88x9240xc2x0 C. to 50xc2x0 C. which is the typical range of use of eyeglass frames. At the same time they maintain the possibility to be plastically deformed in order to be adapted to the unique morphologies of the wearer""s face.
Alloys used in current eyeglass frames production are generally stainless steel, copper based alloys or nickel-copper and nickel-silver due to their easy formability and processing. A recognized limit of these alloys is, on the other hand, their poor elasticity. During normal use they can be easily bent in a permanent way causing distortion of the geometry of the frame. These permanent deformations bring also to a general discomfort for the wearer as the weight distribution on the face is not even and the correct distance between lenses and eyes is lost.
According to previous considerations a lot of attention has been drawn to the development of eyeglass frames with higher elasticity. The idea to use shape memory materials to solve above reported difficulties has been suggested in many articles and patents. Shape memory alloys have the unique property to withstand large deformation without permanent (plastic) deformation. As a figure of merit it can be stated that the maximum deformation sustainable by a shape memory metal, when compared with traditional metal, is about ten times larger. This figure of merit changes with materials and their thermo-mechanical processing but it well justifies the interest for these materials. The first application for the use of shape memory alloy in eyeglass frames dates back to 1979. Since than many applications and patents have been presented. The reason for this long list of application has to be searched in the complex nature of the mechanism underlying the shape memory effect and the pseudoelastic properties of shape memory alloys.
Shape memory alloys are metallic materials in which a solid state transformation takes place. The transformation is called Thermoelastic Martensitic Transformation (TMT) and it is a thermodynamic first order transformation, solid-solid, that can be promoted by temperature changes or by application and removal of a stress field (than the name of thermoelastic). The TMT transforms a generally ordered crystallographic cubic structure (usually referred as Austenite) to a lower symmetry crystallographic structure usually referred as Martensite. It is quite difficult to explain in detail the mechanism underlying the capability of shape memory alloy to restore their initial shape but, for our purposes, it can be simply sketched according to FIG. 1. When in a shape memory metal the thermoelastic martensite forms the crystallographic arrangement of the atoms change from the cubic ordered structure to a complex shape with many internal interfaces (usually referred as twinsxe2x80x94FIG. 1(b)) that has the unique property to arrange very easily. Once a deformation is imposed to the martensite these interfaces tend to expand and to accommodate in order to minimize the deformation at the microscopic level (FIG. 1(c)). A macroscopic shape change takes place without causing permanent damage of the internal structure. Hence, from the point of view of the material, at the microscopic level nothing changes and once the metal is heated up again to the austenitic phase it is able to reconstruct quite easily the original macroscopic shape (FIG. 1(d)). The example describes what is generally called xe2x80x9cone way shape memory effectxe2x80x9d.
We mentioned before that temperature or stress might equally activate TMT. If a shape memory metal is submitted to an external stress (e.g. it is pulled), in a proper temperature range, martensite starts to form. The deformation mechanism is completely equivalent to the one described before so that once the stress state is removed the shape memory metal will recover its initial shape. It is clear that this case exemplifies the xe2x80x9cpseudoelastic effectxe2x80x9d.
By energetic consideration and due to microstructural mechanisms active during TMT both the transformations induced by temperature or the one induced by stress exhibit hysteresis. FIG. 2 compare two examples of a complete thermodynamic cycle in the strain-temperature (xcex5, T) plane (FIG. 2a) and in the stress-strain ("sgr", xcex5) plane (FIG. 2b). They are the experimental evidence of a transformation cycle induced by temperature (T) and of a transformation cycle induced by stress ("sgr"). The two graphs exemplify the shape memory effect and the pseudoelastic effect. In FIG. 2(a) a shape memory metal is cooled causing the TMT. The transformation from austenite to martensite takes place between Ms (martensite start temperature) and Mf (martensite finish temperature). No deformation takes place spontaneously but the material, in the martensite phase, is able to withstand large deformations without microstructural damages. Strain is applied in the martensitic phase causing apparent permanent deformation to set in. By heating the sample, the transformation from martensite to austenite takes place between As (austenite start temperature) and Af (austenite finish temperature). On entering this temperature range the sample starts to recover the initial shape.
In FIG. 2(b) a shape memory metal is pulled at a constant temperature. This temperature must be in the range Af, Md where Af is the previously defined Austenite finish temperature whilst Md is the maximum temperature at which martensite can be induced by an applied stress. As a rule of thumb the temperature range [Af,Md] is generally 10 to 50xc2x0 C. wide according to alloy selection and material processing. On increasing stress level (that is somehow equivalent to decreasing temperature in previous case) martensite start forming in the sample. The TMT taking place is proved by the constant stress deformation taking place at "sgr"L (loading stress). At the end of the plateau all the material is transformed to the martensitic phase that is stable only at that specific stress level. Removing stress (unloading) the TMT takes place from martensite to austenite at a lower stress level "sgr"U (unloading stress).
Both the shape memory effect and the pseudoelastic effect have been proposed to be used in the production of eyeglass frames. In the first case the frame will behave in the following way. Should the temple be accidentally bent, heating it in the temperature range where austenite is stable can restore its original shape. In the second case the frame will behave like an elastic frame. If bent the shape memory metal will accommodate the deformation by forming martensite and, at the very moment the force causing deformation is removed, it will recovery the original shape.
For thermodynamic arguments the temperature range in which the shape memory metal is able to exhibit the pseudoelastic property is quite narrow and it is generally accepted that it is too narrow to be useful for the production of frames. Patents have been proposed to overcome this limitation.
In the above description we have named the shape recovery in the case of the TMT activated by stress as xe2x80x9cpseudoelasticityxe2x80x9d. It is however easy to demonstrate that many times in literature this term is used interchangeably with the term xe2x80x9csuperelasticityxe2x80x9d in order to emphasize the exceptional elasticity degree of these materials. The term xe2x80x9cpseudoelasticityxe2x80x9d should be considered the more precise from a scientific point of view as it points out that the mechanism underlying the macroscopic elastic behavior is not the standard elastic mechanism (Hook""s law). It is a different mechanism related to the presence of the TMT and as a consequence the macroscopic evidence is correctly referred as xe2x80x9cpseudo-elasticityxe2x80x9d, i.e. not the proper usual elasticity.
A further point needs to be clarified in order to fully understand the present invention. It is known to prior art (G. R. Zadno, T. W. Duerig in xe2x80x9cEngineering Aspects of Shape Memory Alloysxe2x80x9dxe2x80x94Butterworth-Heinemann, 414-419,1990) that by cold drawing a NiTi alloy it is possible to obtain enhanced elastic properties without any evidence of the martensitic transformation. In this case the stress strain property will be similar to the one reported in FIG. 3(a). No evidence is present of the pseudoelastic constant stress plateau and the deformation takes place in a roughly linear way. Furthermore this behavior is only slightly temperature dependent and it is not related to the degree of cold work. In this case the effect is referred to as superelasticity or linear superelasticity. Nomenclature choice in this case appears more adequate. In this case the material behaves like enhanced elastic one and there is no evidence of the TMT.